1. FIELD OF THE INVENTION
The present invention relates to electrical circuit boards and more particularly to an assembly for such boards that includes the mounting of integrated circuits.
2. BACKGROUND ART
Current usage of integrated circuitry and printed wiring boards provide high density packaging of electronic circuits that has proved useful not only in saving space but also in reducing signal path length to increase the speed of circuitry. This is particularly valuable in the construction of high speed computers and similar circuitry. A number of techniques have been developed for forming integrated circuit electrical assemblies by combining circuit boards with various components including integrated circuits and the like. Particularly where integrated circuits are utilized it is possible to form assemblies of the type which include a large amount of circuitry in a relatively small package.
Several problems develop in connection with dealing with integrated circuits such as alignment and interconnection of the various leads with a particular problem being presented in certain approaches involving the use of connection leads in the form of wires which must be connected at both ends of the elements to be interconnected. To overcome this particular difficulty certain prior art techniques have been developed utilizing beam leads which extend from overlying etched circuit boards. These beam leads provide connections to various components to be connected directly to the etched circuit board.
Another major problem is that of heat dissipation. Particularly where the dissipation of heat from circuit devices located internally of a portion of a multilayer printed wiring board. Accordingly, adequate cooling is required so the circuit devices will perform without a performance degrading temperature being present. Obviously, a need is developed for a considerable amount of cooling, particularly in connection with the design of high density circuitry. Clearly, the cooling arrangement must be compact so that the advantages in miniaturized circuitry are not overcome by the necessity for providing adequate cooling arrangements.
Yet further problems are developed when integrated circuit chips are mounted onto multilayer printed wiring board structures by means of such techniques as tape automated bonding or flip chip dice attachment techniques. A better realization of the problems can be had by the three techniques that are required in connection with discussion of the present invention. These techniques include multilayer printed wiring board structures, tape automated bonding, and flip chip packaging structures. The multilayer printed wiring board typically is a laminated structure composed of several layers of etched conductor patterns supported by a backing material. These layers are aligned to each other and are laminated under temperature and pressure.
Tape automated bonding is a method to gang bond all bonding sites of an integrated circuit onto an etched conductor site supported by a backing. In this arrangement, these etched sites are then repeated along the length of the same backing material to allow for many dice to be bonded onto a continuous strip. This strip of backing material is referred to as tape. The bonds are formed by forming a eutectic alloy between the etched beams and the integrated circuit chip, or by using thermal compression. This bonding tape is then rolled onto a spool so that the bonded dice may be automatically handled for testing and final assembly.
Finally, flip chip is a method of attaching dice which require a reflow operation. The bonding sites of the dice are solder plated and are then formed into solder bumps. The dice are then assembled to the final substrate by placing the dice face down onto the substrate's conductor network and reflowed soldered.
In general, the above two methods of attachment are used to mount dice onto the surface of a substrate of both glass epoxy and ceramic. Each of the above outlined methods has a certain number of restrictions or problems which restrict their use. The application of tape automated bonding devices relies on the fabrication of a custom tape, so that the etched conductor pattern matches the bonding pads of the integrated circuit. When using the flip chip technique the chip is subjected to problems of thermal expansion and heat dissipation. The method of packaging being disclosed herein makes use of the advantages of both techniques.